Demand for smaller, higher performance, simpler and cheaper antennas continues to increase. The demand is due to multiple factors. One is that terminals for satellite communications and other wireless applications are becoming smaller. Another factor is that crowding of antennas continues to increase, both in space and frequency, increasing demand for improved antenna selectivity, in polarity and frequency. Further, power budgets are becoming tighter, which increases demand for higher transmitter/antenna efficiency. Further, particularly for hand held devices—as these tend to move relative to human bodies—demand for antennas that do not require a separate ground plane, and/or that do not require sharing of other components for an effective ground plane is increasing.
Many of these demands have been met, for approximately the last three decades, by related art fractional-turn quadrifilar helical antenna (QHA). As known, related art QHA have circular polarization, good ground-plane independence, a typically acceptably low backlobe, and a reasonably small size.
Related art QHA are known and, therefore, a detailed description of their theory of operation is omitted. Various structures of related art QHA are also known and, therefore, a detailed description of each is omitted. One example typical related art QHA has two spatially orthogonal bifilar helix loops that are balun-fed, typically at one end, and the helix loops being fractional turn (one-fourth to one wavelength) and having a large pitch angle. The helical elements of related art QHA are open or short circuited, typically at the end opposite the feed end, depending on whether the elements are multiples of one-quarter or one-half wavelength, respectively. The radiation pattern of related art QHA is off the end of the antenna, in a broad beam, cardioid shape.
In one related art QHA arrangement, the feed passes through the central axis of the cylinder supporting the conducting arms to drive the helical arms from the top of the QHA. The radiation in this arrangement is in a direction behind the feed, hence the name backfire antenna.
The theory and structure of prior art QHA is known and is described in many publications available to persons skilled in the art. See, for example, R. C. Johnson, “Antenna Engineering Handbook,” Third Edition, John Wiley, pp. 13-19 to 13-20 (1993).
FIGS. 1-3 illustrate examples of prior art QHA structures and FIGS. 4 and 5 illustrate examples of prior art QHA feeding circuits.
Referring to FIG. 1 shows a prior art QHA arrangement disclosed in U.S. Pat. No. 6,369,776, issued Apr. 9, 2002 to O. Leisten et al. (“the '776 patent”), with reference numbers added, comprising a feeding region arranged as the depicted region 1, a ceramic core (not labeled) having a height, an integrated balun 2 formed of an outer feed conductor 4A connecting to a first pair (not separately numbered) of helical radiating element arms 3A, and having an inner feed conductor 4B connecting to a second pair (not separately numbered) of helical radiating element arms 3A. The helical radiating elements 3A are not equally spaced. The two pairs of helical radiating element arms extend downward (relative to the orientation of the Figure) an axial length from the feeding region 1 to a shorted arms region 3B. A coaxial antenna feed connection 4C extends from the bottom of the core and up through a center portion (not shown) of the core, exiting at the feeds 4A and 4B. The present inventors observe it is known in the art that the electrical path of the elements 3A is λ/2 at the operating wavelength, and known that the balun electrical length is ˜4 at the operating frequency of the antenna. The present inventors have further identified that the height of the FIG. 1 prior art antenna is driven, in part, by the size of the balun.
Prior Art FIG. 2 shows another example prior art QHA, which is taught by U.S. Patent Publication US2006/0082517A1, naming S. Chung and Y. Wang as inventors, showing a U.S. filing date of Nov. 17, 2005 (“the Chung et al. '517 application”). Referring to FIG. 2, this prior art QHA has core material 5, shorted arms region 6, helical windings 7, two of the windings 7 having a thinner width line section, or indentation 9, and a perpendicular balun board 8. As taught by the Chung et al. '517 application, the indentation 9 must be formed only in one of the pairs of helical arms, and is structured to establish a phasing between the arms to create circular polarization. The indentation 9 inherently decreases conductor radiation resistance, even if formed at a minimum current location as taught by the Chung et al. '517 application, and therefore decreases the antenna efficiency.
Prior Art FIG. 3 shows still another prior art QHA arrangement, disclosed as prior art in U.S. Pat. No. 6,535,179, issued Apr. 18, 2003 to A. Petros (“the Petros '179 patent”) having folded arms, arranged and structured as illustrated by the helical arm portions 10, 12, 13 and 14 and their respective parallel folded sections 16, 17, 18 and 19, as labeled by the prior art FIG. 3 of this disclosure. The end of each helical arm portion 10, 12, 13 and 14 opposite its respective parallel folded section (16, 17, 18 and 19) is coupled to a hybrid phase shifter such the related art example illustrated by FIG. 4. The length of the FIG. 3 prior art folded arms, however, is well known to persons skilled in the art as being 3λ/4. See Petros and S. Licul, “‘Folded’ quadrifilar helix antenna,” in Antennas & Propagation Society International Symposium Digest, vol. 4, (Boston, Mass.), IEEE, vol. 4, pp. 569-572, July 2001.) Further, although the known practical limit to which the prior art FIG. 3 arms can be folded is 0.5λ, it is also well known that if the length of the folded element (items 16, 17, 18 and 19) is greater than approximately than 0.18λ the interaction between the arms increases such that a practical and acceptable tuning of the antenna is not likely feasible in the known QHA arts.
Prior art QHA, including the examples illustrated in FIGS. 1-3, are typically connected to a feed circuit providing a different phase shift for each helical element. The phase shifts are often 0, −90, −180 and −270 degrees, for circularly polarized radiation. The circular polarization being left handed or right-handed is determined by the sense of the helical windings (e.g., counterclockwise for right-hand sensed circular polarization and clockwise for left-hand sensed circular polarization) and the phase order of the feed excitations.
Prior Art FIG. 4 shows an example prior art QHA quadrature phase feeding network having an input port, labeled 105, and four phase shift output ports, labeled 110-113. The FIG. 4 exemplary prior art feeding network is formed of three 90-degree hybrid couplers, labeled, 106, 108 and 109, and one minus 90-degree shift line to provide a minus 180 degree phase shift between the input 105 of the hybrid coupler 106 and the input (not labeled) of the hybrid coupler 108.
With continuing reference to prior art FIG. 4, one fundamental aspect of the prior art feeding networks represented by the Figure is that the isolated port of the hybrid coupler 108 is resistively terminated through the element labeled 114 to ground and, likewise, the isolated port of the hybrid coupler 109 is resistively terminated, through the element labeled 115, to ground. Stated differently, prior art QHA feeding networks do not have a differential termination between the hybrid couplers 108 and 109.
Prior Art FIG. 5 shows a block diagram of a second example prior art quadrifilar feeding network, labeled 117, consisting of four separately configured matching networks, labeled 118a-d, two 90-degree hybrid couplers, and one 180-degree coupler. The feeding network 117 typically uses stripline or microstrip or a combination of the two in a distributed series formation. One problem with this arrangement is that the characteristic impedance of the series distribution changes for each phased output and, therefore, each of the four antenna matching networks 118a, 118b, 118c, and 118d must be differently configured.
Referring again to FIG. 5, one fundamental aspect of such prior art feeding networks is the isolated port of the 90-degree hybrid coupler outputting the 0 and −90-degree feeds is resistively terminated, through element 119, to ground. Likewise, the isolated port of the 90-degree hybrid coupler outputting the −180 and −270 degree feeds is resistively terminated, through element 120, to ground. Stated differently, in the FIG. 5 feed mechanism, and in all similar and related prior art feed mechanisms known to the present inventors, there is no differential termination between the different antenna elements fed example 90-degree hybrid couplers.
All of the FIG. 1-5 other prior art QHA have fundamental limitations, though, that will likely pose significant problems as demand for smaller size, higher performance antennas increases. One problem is bandwidth. The bandwidth of prior art QHA is typically narrow, for example 0.25% using a high dielectric constant of, for example, 39 as a representative number. Another problem is size. Prior art QHA, when first introduced, provided size reduction over certain other antenna types, but further reduction in QHA size appears elusive. Incremental improvements have been made, basically due to general improvements in materials sciences and manufacturing methods.
Dielectric loading has been considered for reducing QHA size. The theoretical basis is that, ideally, in an infinite medium the effective wavelength is reduced by a factor inversely proportional to the square root of the relative dielectric constant. Therefore, theoretically, a relative dielectric constant of 25 yields a calculated size reduction factor of five, which is significant. There are fundamental problems, however, with this method. One is that only the core of the antenna can be dielectrically loaded. Otherwise the structure implementing the loading itself increases overall antenna size. Therefore, the size reduction actually attainable with dielectric loading in prior art QHA is much less than the theoretical reduction factor. Cost is also increased. In addition, loss is increased, reducing efficiency and gain. Further, in the prior art QHA the higher the dielectric constant, the higher the Q, and the bandwidth is therefore reduced.
For these and other reasons, a QHA is needed that provides further size reductions, substantial increase in performance, and improved manufacturability.